Gazing Up At A Double Sun

If you headed outside this Labor Day weekend, besides seeing that second blue moon of the month, just look up at the sky, would you believe that about half of those stars you see are actually two stars or more, the kind of double star system that's quite common? And this week, astronomers reported on the discovery of a planetary system orbiting such a binary star, two planets orbiting two suns. It's called Kepler-47 after the Kepler planet-hunting mission that spotted it.

Joining me now to talk about the find is Jerome Orosz. He is the lead author on the paper of the discovery published this week in the journal Science. He's associate professor of astronomy at San Diego University. Welcome to SCIENCE FRIDAY.

DR. JEROME OROSZ: Thank you for having me.

FLATOW: When you say two planets orbiting a pair of stars, a lot of people might imagine some kind of figure-eight, wacky pattern that's going on here.

OROSZ: Yes. It's quite common, and unfortunately, that's not really possible. And so instead what we have is the two main bodies in the system and stars. You know, they have much the mass, and so they have an orbit of about seven and a half days around each other. And then surrounding those two stars is the inner planet called Kepler-47b. Its orbit surrounds the two stars. Its orbital period is about 49 and half days. And then surrounding that orbit is Kepler-47c. Orbital period there is about 303 days. And so if you were to look at this system from above, you'd see sort of nested orbits.

FLATOW: Yeah.

OROSZ: The binary star is inside within the first planet and in the second planet.

FLATOW: So you have two stars that are sort of orbiting themselves, and around those two stars are two planets.

OROSZ: Yes.

FLATOW: What would you see from the surface of the planet? What would a typical day of the suns, the two stars passing overhead look like?

OROSZ: Well, in this case, the second star is about 1 percent the brightness of the other star. And so if you're on that planet, you'd see that the main star - which is not too different than our sun. It would rise and set each day. And then near it, you know, not too far of in the sky, roughly half of your - if you hold your fist down at your arm's length, roughly half that size would be the separation. And so you'd see this dimmer companion sometimes trailing, sometimes leading. Every week, you'd see the two stars eclipse each other. And so it'd be a very interesting show.

FLATOW: So there'd be a time when you'd see one sun because there was an eclipse of the other one?

OROSZ: That's correct, yeah. So every seven and a half days, the larger star would completely block the smaller star. So you'd - for a couple of hours, you'd have a single sun.

FLATOW: So that'd be like one star is in retro grade?

OROSZ: Yes. And so yeah, the directions can change, exactly. So one star can lead the other, then it can trail, then it can lead again.

FLATOW: Must make for an interesting sunrise, too, I would imagine.

OROSZ: Exactly. And so your daylight would be more than half of the rotation period. And so just for the sake of argument, let's say, the rotation period is 24 hours, there will be more than 12 hours daylight, because when one of the stars set, you'd still have the other one up and then vice versa. So one of the stars could rise before the other.

OROSZ: That's correct. And so depending on how you do the statistics, roughly half of the stars you see at night are in multiple systems or either pairs or even groups of three, four, five, six even, right? So the star like the sun that's single is actually sort of an exception.

FLATOW: And how do you discover the pair of stars?

OROSZ: Well, in the case of Kepler-47, we see the orbits of the two stars basically edge-on. And so every seven and a half days, we see a fairly deep eclipse when the smallest star passes in front of the bigger star. And so we measure a 15 percent decrease in the light. Now, backing up, I should say the Kepler telescope, what it does, it simply measures brightness of about 160,000 stars versus time. And it does that round the clock, you know, 24 hours a day and seven days a week. And so when the stars eclipse each other, we can see those brightness dips pretty easily. And so that's why we were looking at Kepler-47, because it was an eclipsing binary.

FLATOW: Wow. 1-800-989-8255 is our number. Talking with Jerome Orosz, associate professor of astronomy at San Diego State University. A paper in science about a binary star system that has two planets going around it. Now, in order for you to see the planets going in front of the star, there has to be sort of a dip in the brightest, correct?

OROSZ: Yes. So we see the orbits of the planets basically, again, from an edge-on perspective. And so once in orbit, the star or the planet passes in front of the stars. It's sort of like what Venus did back in June, you know, it transited across the face of the sun. And so there was roughly a tenth of a percent drop in the brightness of the sun.

FLATOW: And you can see a tenth of one percent drop?

OROSZ: Yes, even smaller. So the Kepler telescope was designed to actually measure dips of the size because its mission is to find transits of Earth-like - Earth-sized planets. And so in favorable cases, they can measure dips of, say, 50 parts-per million, which is pretty amazing.

FLATOW: That's amazing. But, of course, the planets you're finding have to be ones that cross the face of the star you're looking at?

OROSZ: That's exactly right. And so if you just draw the geometry, the odds at this orbital configuration course is actually fairly small. And so roughly speaking, for every transiting system like Kepler-47 that you see, there is probably at least 100 or more systems that you don't see. Because if you're looking on a face-on configuration, you would never see the eclipses of transits, and so there'll be no dip in brightness. And so you just do the statistics, there is, you know, easily a few million or more of these systems and galaxies.

FLATOW: Wow. So what are the odds of finding one with the, sort of, planet sweet spot in there that might have life on?

OROSZ: Yeah, inhabitable. That's actually a good question. It's probably one in the couple 100 if you just sort of draw out angles. And so the further the planet gets from its star, the more precise your alignment has to be for the transit. And so the longer and longer the period gets, the less and less likely it is. And so planets that transit - and there are planets, inhabitables, that transit - those are fairly rare occurrences, and so you need to look at lots and lots of stars to see it happen.

FLATOW: So your paper basically proves that there are lots of binary star systems out there and you can detect planets in them?

OROSZ: Yes, that's correct. Yeah. So it's - before the Kepler mission, it was not really a settled question as to whether close binary stars like Kepler-47 could host close-in planets on certain binary orbits, you know, because the planet formation process may have been disturbed by the binaries. And so a single star clearly is different than binary star. In the binary star, you've got orbital motions and chaotic things going on, which is different compared to a single star. And so these certain binary planets that Kepler is finding shows that nature can indeed form planets in close binary systems. Kepler-47 shows that more than one planet can form.

DARYL: Hi. I read that 47c, the outer of the planets, was about the size of Uranus but it might be in that Goldilocks zone. And I was wondering if the size of it would have an issue with the gravity and possible liquid water forming, if we have any way of figuring that out in the next half century, even?

OROSZ: So that's a good question. So we know from the size of the transits, the radius of the planet and it's, you know, roughly like Neptune or Uranus. Now, we can calculate the average temperature based from the distance from the star and other considerations and it's around the freezing point of water or maybe a little bit higher. And so if this planet had a solid surface like the Earth and had enough mass for an atmosphere, it would have liquid water. Now, so the question is, in this particular planet, what would we see?

And so based on some assumptions about the compositions and the temperature, we think, for this particular case, we might actually see water vapor clouds in the upper atmosphere. And so this planet might look a little whitish if you could get up close. With the Kepler data, of course, we can't really tell what the composition would be, and so it's going to take lots of clever engineering and other things to figure out how to do this in the next, you know, coming decades. But that's a very interesting question, you know, what atmospheres would you find in these types of planets.

FLATOW: Do you need new satellites or telescopes, orbiting telescopes or visits(ph) or things like that to get more information? Or do - what we have - kinds of stuff we're using now, is that adequate?

OROSZ: Well, we need sort of the next generation. The problem is with a planet, it doesn't give off much light at all, and so you can't see the planet next to its star. You can - if it passes in front of the star, you can see the different light. And so if you want to actually see light from the planet, that is really a tough going, because of the extreme contrast. And so you need better techniques. And NASA, you know, has some ideas in the work of - works of how to do this, which are all very complex, and I'm not sure it would work for a system the same as Kepler-47.

FLATOW: Mm-hmm. So where do you go from here?

OROSZ: Well, in the immediate future, every three months, I wait for the next data download, and I just keep looking at the binary stars, trying to find more of these systems. And for Kepler-47, in particular, we had a hint that there might have been a third planet. We saw one transit that could not have possibly been due to either the known planets. And so if that transit event was due to another planet, hopefully, it's going to repeat. So - and so every three months, I'm going to, you know, I'll be first in line to get the data and look.

(LAUGHTER)

OROSZ: And so by the time the Kepler mission ends, we hopefully should have, you know, five to seven years of data or...

FLATOW: Wow. Well, we wish you good luck, Jerome, and thank you for taking time to be with us.

OROSZ: Yeah. Thank you for having me.

FLATOW: Jerome Orosz is an associate professor of astronomy at San Diego State University, talking about these new binary planets. Transcript provided by NPR, Copyright National Public Radio.